1. Technical Field
The invention relates to current collectors that are coated with a material that resists corrosion in a lead acid battery environment. The invention also relates to the formation of a corrosion-resistant coating on lead, lead alloy, lead oxide, and other metal surfaces exposed to sulfuric acid environments such as surfaces of lead-acid battery components.
2. Background Art
Lead-acid batteries are the most commonly-used batteries in the world today and represent approximately 60% of all battery sales. Lead-acid batteries find use in such diverse fields as automotive, lighting, power tools, and telephone systems. Lead-acid batteries typically employ two electrodes, a positive lead dioxide electrode and a negative metallic lead electrode. A sulfuric acid solution is used as the electrolyte.
The charge/discharge mechanism of lead-acid batteries is known as the "double-sulfate" reaction. During discharge, both the metallic lead of the negative electrode and the lead dioxide of the positive electrode are converted to lead sulfate. The reverse process occurs during battery charging, namely, lead sulfate is converted to metallic lead at the negative electrode and to lead dioxide at the positive electrode.
Although lead-acid batteries have numerous designs, many batteries employ lead or lead alloys as current collectors for electrodes. As implied by the name, current collectors store current. Typically, these collectors contain a paste-like active material which performs the current collector function. The paste-like active material is used because it lowers the cost of making current collectors. The current collectors may take on a variety of configurations, however, all are designed to mechanically hold the active material. The active material initially takes the form of a paste comprising active lead and lead oxides, water, and sulfuric acid. The paste is mechanically molded into the lead current collector to make a battery plate. The final battery is constructed by interleaving positive and negative battery plates using separators to provide electronic isolation. A more detailed discussion of lead-acid batteries may be found in Linden, Ed., Handbook of Batteries and Fuel Cells, (McGraw-Hill Book Company, New York, c. 1984), the disclosure of which is incorporated herein by reference.
A variety of battery components in numerous battery configurations are exposed to a sulfuric acid environment. In particular, lead-containing current collecting elements take the form of grids, lead spines in tubular batteries, Plante-type electrodes, thin film electrodes, made using lead sheets, or bipolar electrodes that employ lead-containing surfaces which contact sulfuric acid.
Lead-acid batteries are extremely reliable and can be constructed to have long service lives. However, due to the nature of the battery environment, particularly the potentials generated at the positive plate, one of the main battery failure modes is corrosion. During corrosion, the lead current collector reacts with the acidic electrolyte and is converted into lead oxides. These reaction products are less dense than lead in the elemental form. As more reaction products form, the resultant stress in the oxide layers extrudes the current collector, a process termed "grid creep" when applied to grid extrusion. Grid creep is an irreversible mechanical distortion of a current collector such as a battery grid, resulting in separation of the active material and/or physical distortion of the entire battery. In severe cases of grid creep, the battery housing may crack as the current collector is forced against the walls of the battery. To avoid this problem, battery designers must include extra space in the battery housing to accommodate future expansion due to grid creep.
Numerous solutions have been proposed to alleviate the problem of grid creep in lead-acid batteries. One technique involves alloying the lead current collectors with various elements to increase their rigidity, and hence their resistance to grid creep. However, these alloying elements, which typically add strength through formation of a second phase, e.g. precipitates and eutectics, within a lead matrix, increase the susceptibility of the material to corrosion. The design of the current collector itself can also enhance battery life by providing a configuration that promotes uniform mechanical expansion during corrosion. Increasing the thickness of the battery current collectors also increases their resistance to mechanical distortion.
However, these prior approaches to current collector design merely minimize the effects of grid creep. They do not deal with the fundamental problem of current collector corrosion. In U.S. Pat. No. 5,143,806 to Bullock et al., the disclosure of which is incorporated herein by reference, the problem of lead-acid battery grid corrosion is addressed through the formation of a protective layer of barium metaplumbate, BaPbO.sub.3, on the lead battery grid. Barium metaplumbate is a conductive oxide having the perovskite crystal structure. Although barium metaplumbate resists attack by sulfuric acid, barium metaplumbate does decompose over time to form BaSO.sub.4 and PbO.sub.2 in the presence of sulfuric acid.
Another solution to the problem of collector corrosion is proposed in U.S. Pat. No. 5,126,218 to Clarke (the "Clarke '218 patent"), the teachings of which are incorporated by reference. Clarke proposes making the entire collector of a conductive ceramic material, sub-stoichiometric titanium oxide (TiO.sub.x, where x=1.55 to 1.95). Although such collectors do not corrode or degrade in sulfuric acid, they are difficult to fabricate and use because of the brittle nature of the ceramic material.
Therefore, there is a need for corrosion resistant collectors for lead-acid batteries that avoid the previously described problems.